This descriptive epidemiological study is designed to investigate unusual familial coexistence of cancer and congenital neural tube closure defects (anencephaly and spina bifida). A retrospective epidemiological cohort approach will be used to determine whether conceiving a child with anencephaly or spina bifida has any relationship to the parents' subsequent risk of developing cancer. Evidence of a positive association would enhance our understanding of the etiology of cancer in humans, and could have immediate value in developing preventive measures to reduce cancer mortality through early detection of high risk individuals. We are proposing to study the most direct and epidemiologically useful aspect of unusual familial coexistence between cancer and congenital malformations. There are two underlying biological bases for ths study. First, the fact that carcinogenesis and teratogenesis can sometimes be caused by the same agents (i.e. radiation), as well as the fact that carcinogens can sometimes act through teratogenic-like mechanisms (i.e., diethylstilbestrol). Second, the limited available data on familial coexistence of cancer and congenital malformation theoretically implies that the control of abnormal embryological development and the control of malignant cellular change are genetically related. If so, it is likely that evidence of the association can be detected in these family studies.